1. Field of the Invention
This invention relates to a driving apparatus for driving an object to be driven by an electromagnetic force, and particularly to a driving apparatus having position (rotational angle) detecting means for detecting a variation in a magnetic flux by a magnetic detection element to thereby detect the position (rotational angle) of an object to be driven.
2. Related Background Art
As a driving apparatus of this kind, there has heretofore been, for example, an aperture driving apparatus for a video lens. FIG. 1 of the accompanying drawings shows the construction of such driving apparatus. The outside of a rotor 113 constructed of a permanent magnet having its two poles axisymmetrically magnetized is surrounded by two bobbins 114a and 114b, which are formed with coils 115a and 115b, respectively. Also, a yoke 119 formed into a ring-like shape surrounds the outer periphery thereof. The rotor 113 is adapted to be rotated under the influence of a magnetic field created by a direct current being supplied to the coils 115a and 115b.
A coil-like spring 118 is attached to an arm 116, so that a force in a closing direction may act on aperture blades 117a and 117b. The angular position of the rotor 113 is detected by a magnetic detecting element 128 such as a Hall-effect device, and/or the basis of this detection signal, the amount of electricity supplied to the coils 115a and 115b is determined to thereby control the rotation angle of the rotor and as a result, the control of the opening and closing of the aperture blades 117a and 117b which are objects to be driven.
FIG. 2 of the accompanying drawings shows a block diagram of the basic control circuit of the above-described driving apparatus. The difference (deviation) between an aimed signal from the outside and a position (rotation angle) signal detected by a magnetic detection element 141 and outputted through a detecting circuit 142 is obtained by a subtracting circuit 143, and the output of this subtracting circuit 143 is amplified by an amplifying circuit 144 and a voltage is applied to the coil (not shown) of driving means 146 through a driver circuit 145. That is, a voltage corresponding to the difference between the position signal detected by the magnetic detecting element 141 and the aimed signal from the outside is applied to the coil of the driving means 146 until said difference becomes zero.
In such a driving apparatus, the magnetic detecting element such as a Hall-effect device is unavoidably disposed at a position whereat a magnetic variation by the excitation of the coil is created, in order to detect the magnetic flux of the permanent magnet which is a rotor and make it into a position signal for the object to be driven. Accordingly, this leads to the problem that the magnetic variation by the excitation of the coil is detected by the magnetic detection element and a false position signal is outputted.
The magnetic flux generated by the excitation of the coil is coincident with the direction of variation in the magnetic flux detected as the position signal by the magnetic detecting element when the permanent magnet is moved (rotated) by the magnetic flux. That is, the object to be driven is not driven without a predetermined or greater amount of force determined by the friction or the like of the driving portion being applied to the object to be driven, but even when a force less than the predetermined amount is given by the excitation of the coil and the position of the object to be driven does not change at all, a signal indicative of the fact that the position has changed is outputted from the magnetic detecting element due to the magnetic flux created by the coil. This means that a minute amount of positioning of the object to be driven is difficult.